MECA Team

What strategies do trees adopt to stand upright for so long while supporting their own weight and resisting wind? 

To answer this question, our interdisciplinary team, combining biomechanics, ecophysiology and molecular physiology, is using original approaches to study the resistance and resilience of trees facing mechanical challenges.

Context

Several models predict an increase in the intensity of extreme wind events, while daily wind speeds may decrease. These more extreme conditions caused by the climate crisis constitute a major risk for trees, especially when combined with other risks such as droughts, heatwaves and attacks by insects and fungi. To mitigate this risk, one possibility is to leverage trees' ability to increase their resistance to breakage and uprooting, and their resilience once damage has occurred. This requires a better understanding of the sensed stimuli and of the biological response mechanisms involved.

Objectives

For more than 15 years, the MECA team has been dedicated to an original approach of integrative physics and physiology, combining experimentation and modelling to elucidate the mechanisms that enable trees to develop their structures in a fluctuating mechanical environment. This work has given us national and international scientific renown. Initially very fundamental, our discoveries were then mobilised in the context of more applied research, in particular by extending our questions to the scale of tree populations. The genericity of our approaches has enabled us to show that many of our results are not restricted to trees but can be generalised to plants. Comparisons between tree and herbaceous species also enable us to better understand the specific characteristics of tree growth.

Our research aims to answer two major scientific questions:

• How do plants sense mechanical signals, whether triggered by wind or other factors (their own weight, soil resistance to root growth, water stress, etc.)? How do these mechanical signals impact the functioning of meristems and cause changes in growth patterns?
• What are the biological mechanisms behind plant resilience to extreme mechanical events (breakage and lodging)?

The first question is essential to understanding how plants acclimate to any stress with a mechanical component. There are two types of stresses: external stresses, primarily wind, and mechanical impedances acting directly on growing tissues. We have shown that wind sensing is a major factor in controlling tree growth, from the seedling to the mature stages, and in the long-term survival of trees. We are now conducting more in-depth studies of repeated stresses under controlled conditions and in natural tree populations. We are also studying growth under impeding constrains (1) at the cambium level – whose growth is constrained by the bark, but in a variable manner depending on its water status and that of the surrounding tissues – and (2) at the root level, where growth takes place in soil with variable impedance, particularly in relation to its water status. We are also attempting to study acclimation to mechanical stress and water stress together, based on the hypothesis that the sensing and/or transduction mechanisms are partly common.

Our second research question is related to resilience after an extreme event, for example when a tree is bent or broken, allowing us to also consider the response to events that exceed mechanical resistance. We are studying the mechanisms by which trees gradually manage to straighten up and regain an upright position. This postural control is active during all phases of development, not just after a traumatic event. It is shared by most plants, which is why our research also applies to other plants, particularly wheat and the problem of lodging, when ears are flattened to the ground by the wind; but also to Arabidopsis to help deciphering key molecular mechanisms.

When recovery is no longer possible, another resilience strategy for trees is dormant buds bursting. These dormant buds are present under the bark along the trunk, and burst in response to extreme events to create new stems or epicormic replacements. We identify which environmental factors determine this emergence from dormancy following a mechanical accident or technical intervention (staking, trellising, arching, etc.).

For these two questions, we rely on integrative multi-scale approaches, from molecules to entire tree, including cells and organs. These approaches integrate several disciplines (molecular biology, anatomy, ecophysiology, biomechanics) and involve extensive dialogue between experiments and models. We design original experimental devices and develop (bio)physical analysis tools, including multi-scale analysis of spatial or temporal image series. We characterise biological responses, including the analysis of shape variations, the estimation and control of tissue deformations, the measurement of variations in stem growth in diameter and height, root development, and the anatomical and mechanical properties of the tissues formed. These are combined with research into the molecular actors involved, focusing on key processes. Our results are then interpreted from an agro-ecological perspective, either on forest systems or on more anthropised systems (trees in orchards, urban trees).

The work of the MECA team is gradually shedding light on fundamental questions about tree development and survival. It is also helping to change agricultural and forestry management practices so that they rely more on the acclimation and resilience capacities of trees, thereby improving the overall resistance and resilience of tree systems in their various uses while facing multiple and growing risks.

MECA Tools

• Horizontal microscope
• Gravitron
• Mechanical characterisation tests
• Mechanical deformation control (bending)
• Electrophysiology measurements
• Clinostat
• Interekt